When the size of a document is larger than a recording paper size, the entire image of the document cannot be printed on the recording paper. On the other hand, when the document size is smaller than the recording paper size, an excessive margin in the recording paper may result.
An image forming apparatus, such as, for example, a copying machine, a scanner, or the like, thus typically includes a document size detection device that automatically detects the size of the document. Such document size detection device may include a plurality of reflective optical sensors positioned at predetermined locations in a document plate, and detects the document size based on the signals received from the reflective optical sensors indicative of whether a document to be reproduced is placed on the document plate covering the various location of the respective reflective optical sensors.
Unfortunately, however, because in the above described conventional document size detection device the document size detection is carried out at a predetermined time after the cover starts to close without regards to the speed at which the cover is being closed, a document size detection error can occur due the variance in the amount of available ambient light, and due to the effect of such varying amount of light incident upon the document size detection sensors, depending on the position of the cover at the time of the document size detection.
For example, if the cover is closed at a relatively high speed, the cover may be completely closed before the document size detection takes place, causing small to no amount of ambient light reaching the detection sensors. If, on the other hand, the cover is closed at a relatively low speed, the document size detection may take place while the cover may be still wide open, causing a large quantity of ambient light incident upon the document size detection sensors. Thus a document size detection device capable of mitigating the adverse effect of the variance in the amount of ambient light is desirable.